In a 3GPP system, after a terminal and a network establish secure communication through authentication successfully, a ciphering process is performed between the terminal and an RNC (radio network controller) on the network side. A ciphering mechanism between the terminal and the network side is used to protect data transmitted between the terminal and the network side against thefts or attacks. An input ciphering parameter includes a ciphering key (CK), a ciphering key sequence number (COUNT-C), a bearer identifier (BEARER ID), a transmission direction (DIRECTION), and a required key stream length (LENGTH). If a radio bearer uses a non-transparent RLC (Radio Link Control, radio link control) mode (acknowledged mode AM or unacknowledged mode UM), ciphering is performed at an RLC sublayer; if a radio bearer uses a transparent RLC mode (TM), ciphering is performed at a MAC sublayer (MAC-d entity).
Based on a ciphering key stream generated according to the preceding input parameter, a transmitter performs ciphering computation for a transmitted plain text, and generates a corresponding cipher text. After a receiver generates a deciphering key stream by using the same ciphering parameter and ciphering computation method, the receiver uses the deciphering key stream to decipher the cipher text. In a radio bearer of a non-transparent RLC mode, an uplink and a downlink each correspond to a ciphering key sequence number COUNT-C. The COUNT-C is made up of an RLC HFN (hyper frame number) and an RLC SN (sequence number). High 20 bits of the HFN are initialized with a start value, and the remaining bits are initialized with 0. The HFN increases by 1 after each RLC SN cycle. The terminal transfers the Start value to the RNC through signaling, and each terminal has a Start value list. If a ciphering parameter used by the terminal and the RNC are not synchronized, a data reception error is caused. At present, in UM RLC mode, the detection of UM RLC downlink data transmission errors is implemented by a PDCP layer of the terminal. After the PDCP layer detects a downlink data packet error, the terminal initiates a cell update process, and synchronization of the ciphering parameter may be implemented between the terminal and the RNC through the cell update process.
However, no corresponding detection and recovery mechanism is available for receiving UM RLC uplink data. If packets are lost continuously during the reception of UM RLC uplink data, the uplink ciphering parameter is not synchronized between the uplink transmitting UM RLC entity and the peer receiving UM RLC entity of the RNC. At present, the uplink ciphering parameter can only be synchronized through RRC reestablishment, but the RRC reestablishment causes service interruption and service recovery is slow.